Vapor HF etch process mask and method

ABSTRACT

A method of processing a semiconductor wafer provides a wafer, and then forms an organic mask on at least a portion of the wafer. The method then applies a vapor etching process to the wafer through holes in the organic mask.

PRIORITY

This patent application claims priority from Provisional U.S. PatentApplication No. 60/668,422, filed Apr. 5, 2005, entitled, “VAPOR HF ETCHPROCESS MASK,” and naming John R. Martin and Timothy J. Brosnihan asinventors, the disclosure of which is incorporated herein, in itsentirety, by reference.

FIELD OF THE INVENTION

The invention generally relates to semiconductor fabrication and, moreparticularly, the invention relates to masks used for etching andcleaning.

BACKGROUND OF THE INVENTION

Oxide films are used in a wide variety of silicon semiconductorproducts. For example, oxide layers commonly act as a sacrificial layerfor releasing microstructure in MEMS devices. The art thus uses a numberof different processes to remove or etch oxide layers. One process usesaqueous HF to remove sacrificial oxide under MEMS sensors. Unlesspreventive measures are taken, however, liquid surface tension can causethe MEMS microstructures to stick together (“stiction”) upon removalfrom aqueous baths.

Another wafer fabrication application etches oxides by exposing them toHF vapor (hydrofluoric acid vapor). Such a vapor process is attractivebecause it substantially eliminates the surface tension that causesstiction. For a number of reasons, it often is preferable to apply theHF vapor with portions of the wafer protected by an organic mask. Morespecifically, among other reasons, an organic mask may be more readilyremovable from an underlying surface.

Undesirably, however, prior art organic masks known to the inventorsgenerally cannot sufficiently block HF vapor when used in theseapplications. Specifically, the HF vapor often does not attack/degradethe organic material itself. Rather, it is an ineffective barrier—the HFvapor often diffuses through the organic material. In that case, afterdiffusing through the organic mask, the HF vapor may attack/degrade theunderlying material (e.g., the wafer), causing the mask to debond fromthe reacting surface. Hard (i.e., inorganic) masks are sometimes analternative, but they impose other process limitations. It should benoted that the term “etch” is used herein generally includes chemicalreactions, cleaning and removal of surface films, particulates, andcontaminants, as well as bulk removal of material.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method of processing asemiconductor wafer provides a wafer, and then forms an organic mask onat least a portion of the wafer. The method then applies a vapor etchingprocess to the wafer using the organic mask. The vapor reacts withexposed regions of the wafer that are not protected by the organic mask.

In illustrative embodiments, the vapor etching process includes ahydrofluoric vapor etching process. Moreover, the method may form theorganic mask by depositing a layer of organic material to at least aportion of the face of the wafer, and then forming a supplemental mask(having at least one hole) on the organic material. The method then mayform at least one hole through the layer of organic material by means ofthe at least one hole of the supplemental mask. Alternatively, themethod may form the organic mask by exposing the organic mask to lighthaving a prespecified pattern and then developing the organic mask.Alternatively, other techniques, such as laser ablation and lift-offprocesses may be used to pattern the mask.

The mask may be formed from certain organic materials, such as aparylene or a polyimide. Some embodiments further process the wafer,such as removing at least a portion of the organic mask. In yet otherembodiments, the wafer may be provided by forming a MEMS device on thewafer and/or forming circuitry on the wafer.

In accordance with another aspect of the invention, a method offabricating a micromachined product forms microstructure that issupported by a wafer, and then applies an organic material to at least aportion of the wafer. The method then produces a prespecified pattern ofholes in the organic material to form an organic mask, and applies avapor phase etching process to the wafer through the holes in theorganic mask.

In accordance with other aspects of the invention, a method offabricating a semiconductor device provides a wafer, and deposits alayer of organic material on a surface of the wafer. The method thenforms a supplemental mask (with one or more holes therethrough) on thelayer of organic material, and uses the supplemental mask to form one ormore holes through the layer of organic material to form an organicmask. With this mask in place, the method applies a vapor etchingprocess to the wafer through holes in the organic mask.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages of the invention will be appreciated more fullyfrom the immediately following further description thereof withreference to the accompanying drawings.

FIG. 1 schematically shows a partially cut-away view of a system havinga MEMS device that may be produced in accordance with illustrativeembodiments of the invention.

FIG. 2 shows a process of fabricating a semiconductor device, such asthat shown in FIG. 1, in accordance with illustrative embodiments of theinvention.

FIG. 3 schematically shows a semiconductor wafer are having a pluralityof devices formed thereon.

FIG. 4 schematically shows a cross-sectional view of a partiallyfabricated MEMS device having an organic mask layer on its top surfaceat step 202 of the process of FIG. 2.

FIG. 5 schematically shows a cross-sectional view of the partiallyfabricated MEMS device having an organic mask layer and photoresistlayer on its top surface at step 206 of FIG. 2.

FIG. 6 schematically shows a cross-sectional view of the partiallyfabricated MEMS device having an organic mask layer with throughholesand a photoresist layer on its top surface at step 208 of FIG. 2.

FIG. 7 schematically shows a cross-sectional view of the partiallyfabricated MEMS device having a photosensitive organic mask layer withthroughholes at step 212 of FIG. 2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a semiconductor fabrication process uses anorganic mask in a vapor etching process to etch an underlying material,such as oxide. To that end, the process may 1) apply an organic materialto a surface of a wafer, 2) form a prespecified pattern of holes throughthe organic material to effectively transform the organic material intoan organic mask, and 3) apply a vapor etching process to the waferthrough holes in the organic mask. Among other types, the vapor etchingprocess may be a hydrofluoric acid vapor etching process. Details ofillustrative embodiments are discussed below.

FIG. 1 schematically shows, in a partially cut-away view, an exemplarysystem 10 having a semiconductor chip 13 that may be formed inaccordance with illustrative embodiments of the invention. Specifically,the system 10 includes a packaged semiconductor device 12 having asemiconductor chip/die 13 secured within a conventional semiconductorpackage 14. The package 14 is coupled with a circuit board 16 havinginterconnects 18 for electrically communicating with an external device,such as a computer. In illustrative embodiments, the chip 13 is a MEMSdevice and thus, the packaged semiconductor device 12 also is referredto herein as a “packaged MEMS device 12.”

The semiconductor chip 13 may implement any conventionally knownfunctionality commonly implemented on a MEMS device, such as an inertialsensor. Such a MEMS device 12 may have structure only, or both circuitryand structure. For example, the packaged MEMS device 12 may be agyroscope or an accelerometer. Exemplary MEMS gyroscopes are discussedin greater detail in U.S. Pat. No. 6,505,511, which is assigned toAnalog Devices, Inc. of Norwood, Mass. Exemplary MEMS accelerometers arediscussed in greater detail in U.S. Pat. No. 5,939,633, which also isassigned to Analog Devices, Inc. of Norwood, Mass. The disclosures ofU.S. Pat. Nos. 5,939,633 and 6,505,511 are incorporated herein, in theirentireties, by reference.

Although the semiconductor chip 13 is discussed above as a MEMS deviceimplemented as an inertial sensor, principles of illustrativeembodiments can apply to other semiconductor devices and MEMS devices,such as pressure sensors and microphones. Accordingly, discussion of aMEMS inertial sensor is exemplary and not intended to limit the scope ofvarious embodiments of the invention. Illustrative embodiments thereforemay be used to form other semiconductor devices, such as digital signalprocessor integrated circuits/chips and microprocessor semiconductorcircuits.

FIG. 2 shows a process of forming a semiconductor device, such as theMEMS die 13 within the package 14 shown in FIG. 1, in accordance withillustrative embodiments of the invention. The process began to step200, which begins forming structure, circuitry, or both on asemiconductor wafer. To that end, in illustrative embodiments, asemiconductor wafer may be etched, and/or materials may be deposited orgrown thereon, to begin forming the basic structures of the chip 13. Forexample, a silicon wafer may be selectively etched, and preliminarilyreceive sacrificial material and polysilicon 28 to begin fabricating aMEMS device. As noted above, however, step 200 simply has preliminarystructures. Alternative embodiments, however, may already have somesubstantially completed circuitry and structure.

As an example, FIG. 3 schematically shows a top view of a silicon wafer20 having nine areas 22 for forming separate semiconductor chips 13.Each area 22 may have the beginnings of structure and/or circuitry(shown generally at areas 22) that ultimately form their respectivefinal chips 13. It should be noted that various embodiments may formmore than nine sets of structure and/or circuitry. Accordingly,discussion of nine sets of the structure and/or circuitry for formingnine chips 13 is illustrative. Fewer or more sets therefore may beformed.

After beginning to form structure and/or circuitry, the processcontinues to step 202, which deposits an organic material 32 on at leasta portion of the top surface of the wafer 20 shown in FIG. 3. In someembodiments, a primer, such as A-174 silane, may be deposited first toimprove adhesion of the organic material 32. For illustrative purposes,FIG. 4 schematically shows a simplified, cross-sectional view of one ofthe nine areas 22 of the wafer 20 shown in FIG. 3 immediately afterperforming this step. Specifically, the portion of the wafer die shownin FIG. 4 ultimately will form a single MEMS chip 13.

To that end, the chip 13 comprises a silicon substrate 24 supporting asacrificial oxide 26 and a patterned layer of polysilicon 28.Conventional surface deposition processes and etching techniquespreviously deposited both the oxide 26 and polysilicon 28 on the siliconsubstrate 24 shown in FIG. 3 (step 200). In this example, conventionalprocesses also previously etched the polysilicon layer 28 in a mannerthat forms an unreleased movable member 30 (also step 200). As known bythose skilled in the art, the movable member 30 can be released by asubsequent step that removes at least a portion of the oxide layer 26.Details of the releasing step are discussed below.

In accordance with illustrative embodiments of the invention, and asdiscussed above with regard to step 202, the structure of FIG. 4 alsohas a top layer of organic material 32. It is this organic material 32that ultimately will form an organic mask (also identified by referencenumber 32) for etching or removing lower layers of the structure. Inthis case, as discussed in greater detail below, the organic mask 32ultimately will be part of a hydrofluoric acid vapor etch process toremove a portion of the oxide 26, thus releasing the movable member 30.

Appropriate selection of the organic material 32 is important to theproper execution of the fabrication process. Criteria used to select thematerial includes:

-   -   the permeability rate through the material,    -   the thickness of this material layer,    -   the potential reaction of the material of the underlying layers,        and    -   the removability of this material layer from the underlying        layer.

More specifically, as known by those skilled in the art and using theexample above, hydrofluoric acid generally does not chemically attack oretch organic material 32. Instead, it typically diffuses through organicmaterial 32 at some rate. Moreover, when the chemical interactionbetween a gas and an organic barrier material is relatively small, thegas transport rate through the material can be characterized by apermeation coefficient. This coefficient is the product of the diffusionand solubility coefficients of the material. In essence, thecoefficients describe a three-step transport process:

1. Solubility of the gas into the material,

2. Diffusion of dissolved gas through the material, and

3. Desorption of dissolved gas from the material (in this application,it attacks the wafer surface).

Reducing each step enhances masking properties because this processexecutes in a serial manner. The solubility coefficient (steps 1 and 3)is minimized when chemical similarity between the gas and the materialis minimized. The diffusion coefficient (step 2) is minimized by use ofhighly crystalline materials, highly cross-linked materials (minimalfree volume), and chemical dissimilarity. In addition, the overalltransport process is inversely proportional to thickness (diffusionstep).

A force driving diffusion (step 2) is the concentration gradient ofdissolved gas in the material. Accordingly, in illustrative embodiments,the permeation rate may be reduced by using a material that suppressesdesorption by maintaining substrate adhesion. With minimal desorption,the concentration gradient (the diffusion driving force) is reduced. Tomaintain adhesion, it is important to use a material that has anegligible absorption of the gaseous etchant. This is so because, amongother reasons, if gas concentration in the material at the interface isnear zero, then there is not much available to attack the substrate 24,thus preserving adhesion.

Because some moisture commonly is present in the vapor HF process,hydrophobic, non-moisture-absorbing materials with minimal hydrogenbonding capability are preferred (moisture absorption plasticizes thematerial—this affects both the diffusion rate and the solubility of HF).Minimal hydrogen bonding capability also is advantageous in processesthat add alcohol vapor. Other materials nevertheless could be used.

In illustrative embodiments, the organic material 32 is notphotosensitive and may be comprised of a parylene material. For example,illustrative embodiments may use Parylene C.

In other embodiments, the organic material 32 may be photosensitive andcomprised of a polyimide material, which is a class of photoresistmaterial having species that are designed to remain on an underlyingwafer. Such photoresist typically mechanically isolate stress sensitivecircuit elements from the plastic in standard integrated circuitpackages, and generally are designed to be applied at greaterthicknesses than standard photoresists used in semiconductorfabrication. Moreover, such photoresists generally have a better thermalstability, higher chemical stability, and lower moisture absorption. Afull cure generally provides high cross-linked density. In sum, it isanticipated that this class of photoresists should mask hydrofluoricacid vapor (discussed below) better than many other conventionalphotoresists. For example, among others, this embodiment may becomprised of HD8000, which is distributed by HD Microsystems of Parlin,N.J.

As suggested above, the thickness of the organic material layer 32 isselected based upon, among other things, the permeability rate of theorganic material 32, the type of material to be used in the vapor etch(e.g., hydrofluoric acid), and the amount of time required for the vaporetch. Empirical testing should be sufficient to determine an appropriatethickness.

It then is determined at step 204 if the organic material 32 isphotosensitive (i.e., if it reacts to certain wavelengths of light). Forexample, as suggested above, Parylene C is not photosensitive, whileHD8000 (i.e., a polyimide) is photosensitive.

If the organic material 32 is not photosensitive, then the processcontinues to step 206, which adds a supplemental mask 34 to the topsurface of the organic material 32. As discussed below, the supplementalmask 34 ultimately assists in forming a pattern of holes 38 through theorganic material 32. In illustrative embodiments, the supplemental mask34 comprises a layer of photoresist as shown in FIG. 5.

Accordingly, at step 206, the method adds a layer of photoresistmaterial to the top surface of the organic material 32. As suggestedbelow, the photoresist layer should have a thickness that can withstanda subsequent hole etching process (e.g., using an oxygen plasma) in theunderlying organic layer. The process then exposes such photoresistmaterial to a prespecified pattern of light. Such light exposure andsubsequent development effectively forms a set of holes 36 through thephotoresist material, thus producing the supplemental mask 34. FIG. 5shows an example of two such holes 36 in the photoresist layer.Specifically, as shown in FIG. 5, those two holes 36 are verticallyaligned with oxide 26 that ultimately will be removed in a subsequentvapor etching step, discussed below.

The process then continues to step 208, which forms a set of holes 38through the organic material 32. For example, illustrative embodimentsmay apply an oxygen plasma material to the structure shown in FIG. 5. Asknown by those skilled in the art, oxygen plasma effectively penetratesthrough the non-photosensitive layer (i.e., through the parylene in thisexample). Consequently, the oxygen plasma effectively forms a pattern ofholes 38 (through the parylene layer 32) that corresponds of the patternof holes 36 through the photoresist layer 34. FIG. 6 schematically showsa cross-sectional view of the above noted partially fabricated MEMSdevice after this step forms the holes 38 through the organic material32.

In addition, the oxygen plasma also removes a portion of thephotoresist. It is anticipated that this partial removal primarily has athinning effect on the photoresist. It also is anticipated, however,that this partial removal can also have an impact on inner dimension ofthe holes 36 through the photoresist layer 34, as well as the edges ofthe supplemental mask 34. Those skilled in the art should be aware ofthis potential impact and design accordingly. At this stage of theprocess, conventional techniques may remove the remaining photoresistmaterial (step 210).

Returning to step 204, if the organic material 32 is photosensitive, theprocess continues to step 212 by applying light in a prespecifiedpattern to the organic material 32, and then developing it.Consequently, the step forms a prespecified pattern of holes 38 throughthe organic material 32. FIG. 7 schematically shows a partiallyfabricated MEMS device at this stage of development. At this point inthe process, the partially fabricated MEMS device is ready forconventional vapor etching processes to release the structure orotherwise etch specified portions of the device.

The process therefore continues to step 214, which applies aconventional vapor etch to the partially fabricated MEMS device. Inillustrative embodiments, the vapor etch is a conventional hydrofluoricacid vapor etch. Accordingly, using the above example, hydrofluoric acidin vapor form passes through the holes 38 of the organic mask 32 to etchaway much of the oxide 26 of the partially fabricated MEMS device. Asknown by those skilled in art, this etching step effectively releasesthe movable structure. Moreover, the vapor etch may remove residueformed on surfaces it contacts. For example, the vapor etch may removeresidue formed on the movable member 30 during the fabrication process.If not removed, such residue can introduce another source of error intothe overall system 10.

Conventional processes then remove at least a portion of the organicmask 32 (step 216). In alternative embodiments, however, the organicmask 32 may remain in place.

The embodiments discussed generally relate to using the organic mask 32on at least a portion of a single face of the wafer 20. Such embodimentsnevertheless may be extended to using the organic mask 32 on both thetop and bottom faces of the wafer 20, or on all surfaces of the wafer20. Similar processes may be used to execute such processes.

In illustrative embodiments, applying a vapor etching process throughthe holes 38 in the organic mask 32 suggests that the mask 32 protectssubstantially the entirety of the remainder of the wafer 20 (i.e., thatportion of the wafer 20 that is not exposed to holes 38 in the mask). Ofcourse, as noted above, in practice, it is anticipated that there may beedge effects near exposed areas of the wafer 20. Undesired etching ofsuch additional portions of the wafer 20 nevertheless should not beconsidered to imply that the process is etching other portions of thewafer 20. In other words, in illustrative embodiments, only thoseportions of the wafer 20 exposed to the holes 38 are considered to beetched despite the fact that some undesired etching of edges may takeplace.

It should be noted that the process steps discussed in FIG. 2 areillustrative and not intended to be exhaustive. Accordingly, additionalsteps may be taken to fabricate the MEMS device. For example, the wafer20 ultimately will be diced, and individual chips 13 may be tested,circuitry may be further formed on the wafer 20, additional circuitrymay be formed on the wafer 20, and the chips 13 may be capped, and/orpackaged. The steps discussed in FIG. 2 therefore are not intended tolimit various embodiments the invention. Moreover, the order of some ofthe steps of FIG. 2 also may be changed. For example, the organicmaterial 32 may be deposited as a first step in certain instances.

Accordingly, illustrative embodiments improve the efficiency andeffective use of vapor etching in semiconductor fabrication processes.Consequently, such embodiments significantly minimize stiction problemsbecause they provide a viable alternative to wet etch processes.Moreover, illustrative embodiments facilitate use of an organic mask,which generally is more readily removable from its underlying surfacethan an inorganic mask.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of fabricating a micromachined product, the methodcomprising: forming microstructure supported by a wafer; applying anorganic material to at least a portion of the wafer, the organicmaterial comprising parylene; producing at least one hole in the organicmaterial to form an organic mask; and applying a hydrofluoric acid vaporphase etching process to the wafer through the at least one hole in theorganic mask.
 2. The method as defined by claim 1 wherein producingcomprises: forming a supplemental mask on the organic material, thesupplemental mask having a pattern of one or more holes; and forming atleast one hole through the layer of organic material by using thepattern of the supplemental mask.
 3. The method as defined by claim 1further comprising removing at least a portion of the organic mask. 4.The method as defined by claim 1, wherein the microstructure is a MEMSdevice, the MEMS device including a movable structure, and applying ahydrofluoric acid vapor phase etching process to the wafer releases themovable structure substantially without stiction.
 5. A method ofprocessing a semiconductor wafer, the method comprising: providing awafer; depositing a layer of organic material on at least a portion ofthe face of the wafer, the layer comprising parylene; forming a mask onthe layer of organic material, the mask having at least one hole; andforming at least one hole through the layer of organic material by meansof the at least one hole in the mask; applying a hydrofluoric acid vaporetching process to the wafer through the at least one hole in the layerof organic material.
 6. The method as defined by claim 5, wherein thewafer has a top surface and a bottom surface and wherein depositing thelayer of organic material includes depositing the layer on the topsurface and on the bottom surface of the wafer.